The present invention relates to a battery in which a battery device having electrolyte as well as a positive electrode and a negative electrode is sealed in a film-state packaging member.
In recent years, a secondary battery used as a power source of a portable electronic device has been actively studied and developed. Among the secondary batteries, attention is paid on a lithium secondary battery and a lithium ion secondary battery as secondary batteries capable of realizing high energy density. Conventionally, each of each secondary batteries is generally constructed by interposing a liquid electrolyte (hereinbelow, also called electrolyte solution) obtained by dissolving a lithium salt into a nonaqueous solvent between a positive electrode and a negative electrode and accommodating them in a housing made of a metal.
When a hard case cell made of a metal is used, a problem such that strong recent demands of a lighter, smaller, and thinner secondary battery are not sufficiently addressed occurs. As electronic devices are becoming smaller and smaller, a secondary battery is also demanded to have an increased degree of freedom in shape. When a metal hard case cell is used, the demand regarding shape cannot be also sufficiently addressed.
In order to prevent leakage of the electrolyte solution, it is necessary to use a metal hard case cell (a positive electrode cover and a negative electrode can) having rigidity. As described above, when the non-aqueous solution is used, a problem such as leakage occurs. It is therefore proposed to use, in place of the electrolyte solution, a gel electrolyte obtained by making a non-aqueous electrolyte solution containing a lithium salt held by a polymer compound, a solid electrolyte obtained by dispersing or mixing a lithium salt into a polymer compound having ion conductivity, or an electrolyte in which a lithium salt is held by a solid inorganic conductor. This non-aqueous gel polymer secondary battery has a positive electrode having a positive electrode collector on which a positive electrode active material layer is formed, and a negative electrode having a negative electrode collector on which a negative electrode active material is formed and has a structure that a gel layer containing an electrolyte is sandwiched between the positive electrode active material layer of the positive electrode and the negative electrode active layer of the negative electrode.
In the gel layer containing the electrolyte in such a non-aqueous gel polymer secondary battery, an electrolyte solution is held in a gel matrix. By using the gel or solid electrolyte, the problem of leakage of the electrolyte solution is solved. The hard case cell becomes unnecessary. The degree of freedom in shape can be increased by using a film more flexible than a metal housing or the like as a packaging member. Further reduction in size, weight, and thickness can be realized.
In the case of using a film-state case such as a laminated film, a polymer film, or a metal film obtained by covering metal foil made of aluminum or the like with a resin as a packaging member, however, when lithium hexafluorophosphate (LiPF6), lithium tetrafluoroboric acid (LiBF4), or the like is used as a lithium salt, a problem such as a battery expansion occurs. One of the factors of this phenomenon may be considered that, even if a very small amount of moisture exists in a battery system, a lithium salt is descomposed and a free acid component such as hydrogen fluoride (HF) or ion fluoride is generated. When the free acid component reacts with the lithium to form lithium fluoride (LiF) or the like and the lithium in the battery system is consumed, problems such that shelf stability or charge/discharge cycle characteristic deteriorates and a theoretical battery capacity cannot be obtained, occur.
In a conventional secondary battery using non-aqueous gel electrolyte or solid electrolyte, lithium-cobalt complex oxide is used as a positive electrode active material. A secondary battery using a non-aqueous gel electrolyte or solid electrolyte housed in a metal foil laminate case has a significant challenge to suppress expansion which is seen in a high temperature storage test or the like since a housing for accommodating the aluminum laminate pack may be broken due to the expansion.
In a conventional non-aqueous lithium ion secondary battery, the positive electrode active material contains from 0.8% to 1.2% of lithium carbonate (Li2CO3) so as to provide the function of generating CO2 gas to shut down a safety valve in the case where the temperature of the battery becomes high when heated or excessively charged. A conventionally used positive electrode active material includes about 500 ppm of water content by which a gas is generated when the battery is heated or excessively charged.
On the other hand, a non-aqueous gel polymer secondary battery has improved safety against heating and excessive charging, and it is unnecessary to generate a gas when the temperature becomes high. The conventional non-aqueous gel polymer secondary battery uses lithium-cobalt complex oxide as a positive electrode active material. A non-aqueous gel polymer secondary battery using a metal foil laminate pack obtained by covering metal foil such as aluminum foil with a resin has a significant challenge to suppress expansion, which is seen in a high temperature storage test or the like since there is the possibility that an aluminum laminate pack is not housed in a set case due to the expansion.